1. Field of The Invention
This invention relates to transmission line transformers. More particularly, this invention relates to broadband transmission line baluns.
2. Description of the Background Art
A balanced to unbalanced transformer, also called by its contraction “balun”, essentially provides the same characteristics as an isolation transformer that transmits the energy from input to output by a transmission line mode instead of by flux linkages as in the case of conventional transformers.
Baluns allow a grounded source to drive an ungrounded load or a balanced load where the midpoint is grounded. Further, baluns allow phase reversal of the signal. Still further, if a positive signal is applied to its input, the positive output lead could be grounded. If the return current is forced to flow in the shield, then there is no radiated field from the transmission line because, from a point outside the line, both currents are located at the center of the line and cancel each other out. However, a field still exists between the center conductor and the shield.
Baluns in the form of a conductive sleeve, also known as bazooka baluns, are commonly used to match balanced antennas to unbalanced coax feeders. A prior art bazooka balun 10 is shown in FIG. 1 as comprising a conductive sleeve 12 of a quarter wavelength long that is positioned over the end 16 of a 75Ω cable 14 and shorted to the shield 18 of the cable 14 (i.e., its unbalanced line 20) only at its proximal end 22 to reflect an open circuit at its feed point 24 of its inner conductor 26. The balanced line 28 constituting the load Z may comprise, as shown, a half-wave dipole antenna 30. The conductive sleeve 12 may fit tightly around the existing outer plastic mantle 32.
Notably, the bazooka balun's sleeve 12 shorted to the shield 18 forms a second coaxial transmission line (i.e., the sleeve being the “shield” of the new line and the original “shield” being the inner conductor). In theory, a perfect lossless quarter wave transmission line would present an infinite impedance at the frequency where it is a quarter wavelength long. In practice, however, using a small-diameter sleeve positioned tightly over only the cable's existing outer plastic mantle as the dielectric spacer presents a low impedance. In contrast, a significantly higher impedance can be attained by using a dielectric tubular cylinder installed over the coaxial cable to which a larger-diameter sleeve is then installed. A representative patent showing a larger-diameter sleeve includes U.S. Pat. No. 4,737,797, the disclosure of which is hereby incorporated by reference herein. U.S. Pat. No. 6,552,689, the disclosure of which is hereby incorporated by reference herein, discloses a balun in which a conductive sleeve and a dielectric cylinder of the same diameter are stacked and then positioned within a larger-diameter outer dielectric cylinder.
The impedance and the lower frequency end of the bandwidth may be increased through the use of baluns that employ ferrites in lieu of sleeves. Without the use of ferrite, the inductance of the shield to ground limits the low frequency end.
Prior art ferrite baluns employing ferrite beads have often comprised a plurality of ferrite beads positioned over the cable close to the feed point to maintain a high impedance across the frequency band. See for example, U.S. Pat. No. 4,962,359, the disclosure of which is hereby incorporated by reference herein. Other prior art ferrite baluns have employed ferrite sleeves. See for example, U.S. Pat. No. 4,719,699, the disclosure of which is hereby incorporated by reference herein. Finally, still other ferrite baluns have been incorporated into the frame of a Yagi antenna. See for example, U.S. Pat. No. 4,028,709, the disclosure of which is hereby incorporated by reference herein.
Unfortunately, when using ferrite baluns, a parasitic line is formed by the shield to ground. The parasitic effects of employing ferrite at high frequencies is well documented. U.S. Pat. No. 5,296,823, the disclosure of which is hereby incorporated by reference herein, acknowledges such parasitic effects and teaches specific printed circuit and shielded structures for parasitic mode suppression. Specifically, as illustrated in FIG. 11 thereof, one shielded structure includes the side-by-side positioning of a ferrite core having a central passage containing a two-conductor transmission line and a dielectric having a central passage containing a single-conductor transmission line, into an enlarged shielding tubular signal ground conductor. According to its teachings, the shielded structure confines signal energy and prevents coupling to other outside circuits and structures.
There presently exists a need for an improved balun that isolates the balun from the effects of ferrite at low frequencies, thereby significantly reducing losses and improving voltage standing wave ratio (VSWR).
Therefore, it is an object of this invention to provide an improvement which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the balun art.
Another object of this invention is to provide a balun that employs a ferrite to increase the impedance and the lower frequency end of the bandwidth while isolating the balun from the parasitic effects of ferrite at low frequency and thereby significantly reducing losses and improving VSWR.
The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.